We propose to study the genetic system of murine leukemia virus (MuLV) by using temperature sensitive (ts) mutants. The objectives of the project are: 1. To determine how many genes are involved in viral infection and the maturation process, 2. To find out what functions some of these genes control and their interrelationship with one another, and 3. To elucidate the organization of the viral genome. In order to achieve these objectives the following methods will be employed: 1. Isolation of spontaneous or induced ts mutants by a rapid, selective procedure, 2. Sorting out the ts mutants into groups using complementation tests such that each group represents a defect in a particular gene, 3. Characterize the defect of each complementation group using different biochemical and physiological techniques, and 4. Elucidation of the organization of viral genome by recombination studies.